In a wireless network that uses carrier sense multiple access with collision avoidance, CSMA/CA, as its basic access mechanism a receiving station listens to the wireless medium before transmitting, and if the medium is busy, it will not transmit but will wait until the medium is clear. This is so as to avoid collisions in the medium. An example of such a wireless network is one that is based upon the IEEE 802.11 standard, commonly known as Wi-Fi. To meet this requirement a receiving station commonly has two methods of sensing the medium: physical carrier sense and virtual carrier sense. The physical carrier sense senses that there is radio frequency activity above a certain threshold whereas the virtual carrier sense decodes the received transmission and detects a value that is contained in the transmitted signal that is known in the IEEE 802.11 standard as the network allocation vector, NAV. The NAV is a value that indicates to all stations that receive the signal the time that remains before the medium will become free again after the transmitted packet has ended. In the case of a single packet transmission, the NAV allows time for that packet to be acknowledged. The physical and virtual carrier sense mechanisms in practice exert an indication to the station that informs the station that it shall not transmit. This indication is generally known as clear channel assessment, CCA. Hence, if CCA is exerted in a station, then that station will not transmit. IEEE802.11-2007 and all previous versions, specify the received signal levels for CCA to be exerted for any valid signal and for any radio frequency energy level detected. These levels are known as the CS/CCA (carrier sense CCA) threshold and ED-CCA (energy detect CCA) threshold respectively. For example, in general, for 20 MHz channels, any valid signal detected at or greater than −82 dBm shall exert CCA, and any energy level detected at or greater than −62 dBm shall also exert ED-CCA. The figure of −82 dBm is based upon the specified minimum modulation and coding rate sensitivity. The sensitivity threshold of a station can also determine the CCA threshold. If the receive sensitivity threshold of a station is higher than the CS/CCA threshold, for example, the effective CS/CCA threshold will be the receive sensitivity threshold.
A simplified block schematic diagram of the receiver section 10 of a station that uses CSMA/CA is shown in FIG. 1.
The radio frequency signal received at the antenna 11 is amplified by amplifier (such as low noise amplifier LNA) 12 and then converted to digital form by analog to digital converter (ADC) 13. After being mixed by mixer 14 to convert the received frequency to the baseband frequency, the signal is applied to the digital front end block 15. The processes carried out in this digital front end block 15 include amplification by a digital amplifier (digital Amp) 16, applying an automatic gain control scheme by automatic gain controller (AGC) 18, undergo an input signal attribute measurement such as a received signal strength indicator (RSSI) measurement by RSSI module 19, and signal detected by signal detector 17. Note that there is a direct relationship between RSSI and the received signal strength. AGC 18 will measure the radio frequency, RF, energy level detected.
The RSSI indication is effectively a measurement of the received signal level of a valid received signal. The signal detect block will indicate that a received signal, greater than the received signal detect threshold has been received and will indicate that the medium is busy, or that CS/CCA or ED-CCA is exerted, and cause the received signal to be processed. Hence, the AGC 18 indicates the general RF energy detected, and the RSSI 19 indicates the received signal strength of a valid signal. The settings of the relevant thresholds are set using registers 20. The RSSI value is also written to a register which is then read by the medium access control, MAC, block 22. The processed signal from the digital front end 15 is demodulated by demodulator 23 and passed to the media access control (MAC) module 22.
There are three basic thresholds for the signal detector 17: receive sensitivity threshold, CS/CCA threshold and ED-CCA threshold.
The receive sensitivity threshold sets the level of the minimum sensitivity of the receiver, any signal, valid or not, that is at a level below this threshold will not be processed in any way, it is effectively lost or undetected.
The CS/CCA threshold is a value received by the signal detector 17 from the MAC module 22 and sets the level at which any valid signal that is received above this level will exert CCA and declare the medium busy.
ED-CCA threshold is a value received by the signal detect block from the MAC module 22 and sets the level at which any RF energy that is received above this level will exert ED-CCA and declare the medium busy.
As long as the receive sensitivity threshold is set to a level higher than the CCA threshold, then an RF signal that is received at less than this threshold would not be detected and hence the receive sensitivity threshold would also set the CCA threshold. For example, if the CS/CCA threshold is −82 dBm and the receive sensitivity threshold is set to −75 dBm, then the effective CS/CCA threshold would also be −75 dBm irrespective if the CS/CCA threshold was set to −82 dBm. Also, if the CS/CCA threshold were set to −82 dBm, the ED-CCA threshold set to −62 dBm and the receive sensitivity threshold is set to, say −50 dBm, then the effective ED-CCA threshold would be −50 dBm.
Having a fixed CCA threshold can often result in a station being prevented from transmitting even when, in fact, it could transmit without causing any interference to the other station that is the cause of the CCA being exerted. To better explain this situation, four examples are given.
Example 1 is shown in FIG. 2. A station, STA A, 30, is located at a distance D from its own access point, AP1, 31 and another station, STA B, 33 in an overlapping network is located at a distance of 4D from AP1 31 and 2D from STA A, 30.
Indoor RF propagation loss for this type of indoor application can be assumed to be in the order of 10 dB per octave which is a distance factor of about 35 log(d), where d is the distance. Assume that at AP1 31 the signal strength from STA A 30 is −50 dBm. Then the signal strength from STA B 33 which is four times the distance away from AP1 31 than STA A 30, and behind a wall 35 that has a penetration loss of 10 dB, will be in the order of −80 dBm. Hence, with a difference of 30 dB in the relative signal strengths of STA A 30 and STA B 33, AP1 31 can receive a transmission from STA A 30 at the same time that STA B 33 is transmitting. The signal to noise plus interference ratio, SNIR, at AP1 31 is in the order of 30 dB, more than sufficient for good reception. Similarly, in this example, AP2 32 can receive a signal from STA B 33 while STA A 30 is transmitting. Note, however, that in this example, STA A 30 will receive a signal from STA B 33 at a level of about −66 dBm; therefore if the common specified value of −82 dBm is used for the CS/CCA threshold, if STA B 33 is transmitting then that signal from STA B 33 will exert CCA in STA A 30 and STA A 30 will not transmit. The point to be noticed is that in this example, both STA A 30 and STA B 33 could transmit at the same time and their respective access points, AP1 31 and AP2 32 would receive their respective signals without problem, but in order to do this, the CS/CCA threshold or receive sensitivity threshold would need to be set higher, say −60 dBm.
Example 2 is shown in FIG. 3. This example is for an apartment block 40 where any particular apartment is surrounded by other apartments on either side, above and below. The received signal strengths in the home apartment 41, from each surrounding apartment, can be estimated using an empirical formula for indoor propagation loss. Such a formula is that of Erceg et al, 2004, “TGn Channel Models”, IEEE 802.11-03/940 r4. Calculated results for the received signal strengths in the home apartment 41 and the surrounding apartments are shown in FIG. 3. In this particular case the assumed apartment size is 20 by 35 feet. Note that, in this example, assuming a CS/CCA threshold of −82 dBm, a station in the selected home apartment 41 is subject to potential interference from 32 surrounding apartments.
In the 2.4 GHz band there are only three non-overlapping channels, and in the 5 GHz band there are about 20 channels of 20 MHz bandwidth and only 10 channels of 40 MHz bandwidth, depending on different areas of the world, hence the probability of overlap and interference is high. If, however, the CCA threshold or the receive sensitivity threshold were set to −50 dBm, then this would result in a station within the selected home apartment 41 being subject to possible interference from only 4 surrounding apartments, down from 32. Within the home apartment 41 the minimum signal strength is in the order of −38 dBm but note that the highest signal from any apartment other than the 4 immediately surrounding the home apartment, is in the order of at least −60 dBm; a minimum of 22 dB difference. Hence, in this example, if the CS/CCA threshold or receive sensitivity threshold is set to −50 dBm and there are at least 5 channels available, then the home apartment 41 could select a channel and transmit at the same time as any other network in any other apartment. If all apartments had networks where the CS/CCA threshold or receive sensitivity threshold was set to −50 dBm, then because the network in each apartment had a maximum of only 4 overlapping interfering networks, the channel reuse is significantly improved and, for example, each network could operate using an independent 40 MHz channel.
Example 3 is that of the case of terraced houses as shown in FIG. 4.
In this example the positions of the stations, STA 1, 71, STA 2, 72, STA 3, 73 and STA 4, 74, are chosen so as to represent the worst case for interference to STA 1, 71. The received signal strengths at the various devices can be estimated using the Erceg formula for indoor propagation loss. Assuming a penetration loss of 10 dB for the walls, the calculated results for received signal strength for the STAs are:
STA 1, 71, STA 2, 72, STA 3, 73 and STA 4, 74 to respective APs −48 dBm
STA 2, 72, to STA 1, 71 −34 dBm
STA 3. 73, to STA 1, 71 −68 dBm
STA 4, 74 to STA 1, 71 −84 dBm
Note that with the default CS/CCA threshold value of −82 dBm, STA 2, 72, and STA3, 73, would both exert CCA on STA 1, 71, if they used the same channel and that STA 4, 74, may exert CCA at STA 1, 71, periodically. Note that if the CS/CCA threshold of STA 1, 71, were set to −50 dBm, or even −60 dBm, then transmissions from STA 3, 73, or STA 4, 74, would not cause STA 1, 71 to exert CCA and STA 1, 71, could transmit at the same time that STA 3, 73, or STA 4, 74, was transmitting. Note also that in this case, the transmission from STA 3, 73, to AP3, 63, would be successful as the SNIR at AP2, 62, would be at least 20 dB. Furthermore, as STA 1, 71, is at the furthest possible distance from its AP, AP1, 61, note that any station located in the same house as STA 3, 73, could transmit at the same time as any station in the same house as STA 1, 71, with an SNIR of 20 dB or more and hence have a successful communication, and vice versa. However, if the STAs are using the default CS/CCA threshold of −82 dBm, this is not the case and only one station could transmit at a time. As in the previous examples, raising the CCA threshold or the receive sensitivity threshold would allow simultaneous transmissions and increase the potential throughputs of the networks.
Example 4 is shown in FIG. 5 and represents using a 7-cell cluster of networks with an AP situated at the center of each cell.
Two adjoining seven cell structures are shown. Seven different channel frequencies are used, one for each of the 7 cells in each cluster. As shown in FIG. 5, the same channel frequency that is used in the cell where STA A 91 and AP A 81 are located is also used in the cell where STA B 92 and AP B 82 are located. The positions of STA A 91 and STA B 92 are chosen to represent a worst case. Assuming that the radius of each cell is r then the distances between the APs and STAs of interest, using standard geometry, are:
Distance STA A 91 to AP A 81=r
Distance AP A 81 to AP B 82=4.77r
Distance STA A 91 to STA B 92=2.64r
Distance STA B 92 to AP A 81=3.61r
Assuming the propagation loss due to distance is 35 log(d), where d is the distance, and assuming an additional obstruction loss of 3 dB per cell wall, then STA A 91 will receive transmissions from STA B 92 at a level equal to −(35 log(2.64)+9)=−24 dB relative to a signal from AP A 81. Also, AP A 81 will receive a signal from STA B 92 at a level of −(35 log(3.61)+9)=−29 dB relative to a signal from STA A 91. If we assume a cell radius of 40 feet, then the signal strength of the signals between STA A 91 and AP A 81, using the Ecerg formula, is in the order of −50 dBm, and similarly the signal strength of the signals between STA B 92 and AP B 82, is also in the order of −50 dBm. Hence STA A 91 would receive transmissions from STA B 92 at a signal strength of about −50−24=−74 dBm which is high enough to cause STA A 91 to exert CCA and hence prevent both STA A 91 and STA B 92 from transmitting at the same time. If STA A 91 could set its CS/CCA threshold or its receive sensitivity higher than −74 dBm, then STA B 92 transmissions would not exert CCA in STA A 91 and STA A 91 could transmit at the same time as STA B 92 with sufficient SNIR. Similarly if STA B 92 set its CS/CCA threshold or receive sensitivity threshold higher than −74 dBm, then STA A 91 transmissions would not exert CCA in STA B 92 and STA B 91 could transmit at the same time as STA A 91 with sufficient SNIR. Therefore, if the CS/CCA threshold or the receive sensitivity threshold was set at −50 dBm or −60 dBm, and there were at least 7 channels available, then a seven cell cluster network area layout is possible. Using the default CS/CCA threshold a seven cell cluster layout is not possible.
In each of these examples it is shown that practical situations exist where networks on the same channel could be transmitting simultaneously but are prevented from doing so because of the default CS/CCA threshold. To overcome this, the CS/CCA threshold or the receive signal threshold could simply be set to a higher value but, as will be shown later, this does not result in a network coverage area that accommodates all the STAs that are within the desired area. An alternative may be to use transmit power control, TPC. The major problem with TPC is that unless every STA in the network and, more importantly, every STA in all surrounding networks is using TPC, it does not produce the desired effect. In addition if one STA or network uses TPC it puts itself at a disadvantage as it effectively can make itself hidden from other STAs and networks and hence experience problem in competing for the medium. Therefore, there is no incentive for a STA or network to use TPC.